The decision by the charismatic Moaz Khatib highlights how Syrian dissidents have failed to overcome deep differences in their diverse ranks.

BEIRUT — The head of a U.S.-backed Syrian opposition coalition resigned his post Sunday, a major blow to a group that the United States and other nations have lauded as the legitimate representative of the Syrian people and a potential interim government.

The departure of the charismatic Moaz Khatib, a moderate Islamist who has championed national reconciliation, plunged the fractious Syrian dissident alliance into disarray as the escalating Syrian conflict showed fresh signs of spreading instability beyond its borders.

Israeli forces in the occupied Golan Heights reported firing a guided missile Sunday at a Syrian military post, responding to gunfire that struck an Israeli vehicle along the disputed frontier. Clashes have been flaring on the Syrian side, raising the specter that Islamist insurgents could gain control of territory adjacent to a United Nations-monitored border strip that had been relatively calm for decades.

Meanwhile, the government of Lebanon collapsed over the weekend, a victim at least in part of spillover from the war in neighboring Syria. Outgoing Prime Minister Najib Mikati had tried to keep Lebanon officially neutral in the raging conflict next door, but the nation's two major political camps are backing opposing sides in the Syrian war, placing a huge strain on Lebanon's delicate political balance.

And U.S. Secretary of State John F. Kerry made a surprise visit to Baghdad on Sunday, pleading with Iraqi officials to stop allowing Iranian planes to cross Iraqi airspace with arms for the Syrian government. Iraqi Prime Minister Nouri Maliki insisted that there was no proof the flights carried weapons.

The resignation of Khatib again highlights how Syrian dissidents have failed to overcome deep differences in their diverse ranks, which include Islamists and secularists, Arabs and Kurds, armed fighters and nonviolent activists.

Since his appointment in November, Khatib has been the public face of the Syrian exiled opposition. In late February, Khatib met publicly in Rome with Kerry.

In Sunday's statement on his Facebook page, Khatib said he had always maintained he would step down if certain "red lines" were crossed, but he provided no specifics. There was considerable speculation that Khatib objected to his own coalition's decision last week to name a Syrian expatriate from Texas as "prime minister" of rebel-held areas of Syria, a move that may have undermined Khatib's authority.

The rebel Free Syrian Army declared Sunday that it did not recognize the new premier, Ghassan Hitto, a little-known technology executive — yet another sign of discord in the opposition alliance.

Khatib said he planned to continue opposition efforts, working "with freedom that cannot be provided within official institutions."

In his statement, he seemed to express little hope that an end was near for a two-year conflict that has cost tens of thousands of lives and sent more than 1 million Syrians fleeing the country.

"The regime, with its recklessness, has lost the most valuable of chances for comprehensive national reconciliation," said Khatib, who has repeatedly called for harmony among Syria's various religious and ethnic groups.

In announcing his resignation, Khatib also criticized the international community for not providing Syrians with sufficient means to "defend themselves" against attacks from the government of President Bashar Assad.

The White House has refused to arm the Syrian opposition, fearing in part that the weapons could end up in the hands of Al Qaeda-linked militants. Kerry recently promised tens of millions of dollars' worth of nonlethal aid to the opposition, but not weapons.

The Syrian National Coalition, as the opposition bloc is known, has reportedly asked Khatib to stay on despite his very public resignation.

It was no secret that Khatib had clashed with coalition leadership.

Some Syrians have criticized the coalition as a tool of the Muslim Brotherhood, the Islamist organization, and beholden to Qatar, Saudi Arabia and Turkey, which together have bankrolled and otherwise subsidized the anti-Assad forces.

The coalition was formed in November in Qatar as part of an effort to present a united dissident front. The United States and allies calling for Assad's departure applauded the coalition's formation as heralding a new era of opposition unity.

The coalition was supposed to serve as a kind of government in exile, a conduit for foreign aid and an overseer of military operations in Syria. The organization has yet to achieve those grandiose ambitions, however.

The naming of Khatib as the coalition's president was widely hailed. Unlike many other exiled dissidents, he had until recently been living in Syria, where he was a preacher at the historic Umayyad Mosque in Damascus. He also had participated in anti-Assad protests and had been jailed for his activism.

Still, the coalition has remained largely an exile entity; its support inside Syria appears tenuous at best.